PYROMETRY 415 imate value can be obtained by the use of an auxilliary thermometer, the bulb of which is placed about three-fourths of the way down the exposed mercury thread. Tables 4, 5 and 5A show the magnitude of the emergent stem correction in some typical cases. TABLE 5.—STEM CORRECTION DATA FOR DISTILLATION FLASK USED IN DISTILLATION OF CREOSOTE OiL1 Dimensions of flask: Diameter bulb 8 cm., diameter neck 2 cm., height neck 15 cm., distance bottom of outlet tube to top 8 cm. Thermometer range 0 to 400°C. in 1° intervals, length 40 cm. Temperature of bulb, degrees Centigrade Degrees of mercury column emergent Mean temperature of mercury column Stem correction 208 200 59 4.7 258 250 105 6.0 308 300 87 10.5 358 350 79 15.5 TABLE 5A.—STEM CORRECTION DATA FOR PENSKY-MARTIN FLASH-POINT APPARATUS Thermometer used up to 150°C., range 40 to 160°C. in 1° intervals, scale length 9.5 cm. Thermometer used from 200 to 300°, range 200 to 360° in 1° intervals, scale length 12 cm. Temperature of bulb, degrees Centigrade Degrees of mercury column emergent, degrees Centigrade Mean temperature of emergent mercury column, degrees Centigrade Stem correction 50 30 35 0.1 100 80 45 0.7 150 130 55 2.0 200 10 75 0.2 250 60 85 1.6 300 110 100 3.5 Partial Immersion Thermometers.—As a means of avoiding the necessity for applying the correction for the emergent stem, it has become customary to use partial immersion thermometers which are graduated to read correctly when immersed to a definite depth, say three inches, in a bath. Such thermometers should be, and usually are, marked to show the proper depth of immersion, and many of them have a line engraved around the stem to show this depth of immersion. While the accuracy attainable with a partial immersion thermometer is not so high as can be attained with a total immersion thermometer under favorable conditions, it is true that these favorable conditions are not often realized, and therefore in the great majority of cases, the partial immersion thermometer is to be preferred. It is probable that the partial immersion thermometer will be extensively used in this country. * See paper by Wilhelm, Bureau of Standards Tech. Paper No, 49.